1. Technical Field
The present invention relates to charging systems in general, and in particular to a charging system that can maintain a high-setting voltage of a battery charger while maintaining a low-setting value of an overvoltage protection circuit with respect to a battery cell.
2. Description of Related Art
Most rechargeable batteries for portable computers take the form of a battery pack having multiple battery cells composed of a lithium ion rechargeable battery with a high-energy density. Upon charging/discharging the lithium ion rechargeable battery, it is necessary to precisely control charging/discharging current and voltage. In particular, it is necessary to strictly control the charging voltage in a constant-voltage control period. Therefore, a processor is usually provided within a battery pack having lithium ion rechargeable batteries. The battery pack generally employs a smart battery in which a processor monitors an internal state of the battery pack during charging and discharging in order to send information to a portable computer or to activate a protection circuit. The smart battery is a battery device that is compliant with the standards called smart battery system (SBS) initiated by Duracell Inc. and Intel Inc. A battery pack compliant with the SBS standard is also called an intelligent battery.
In an intelligent battery, an electric circuit portion having a processor, a current measurement circuit, a voltage measurement circuit, a temperature sensor, and a rechargeable battery, all mounted on a substrate contained within a housing. The processor is operable to communicate with an embedded controller of the portable computer via data lines. A protection circuit is also installed in the intelligent battery; therefore, when an abnormality has occurred in the cell during use, the protection circuit can be operated to stop charging/discharging operations. As items for estimating the abnormality to be detected by the intelligent battery, information such as current, voltage, temperature, and voltage balance between the battery cells during charging/discharging are included.
In an existing charging system for an intelligent battery, the battery pack and the portable computer include a respective voltage measurement circuit. A measurement value of the voltage measurement circuit incorporated in the battery pack is used to stop charging when the voltage of the battery cell exceeds a predetermined value during the charging process. The voltage measurement circuit incorporated in the portable computer measures an output voltage of the battery charger, and the measurement value is used for feedback control of the battery charger.
When the output voltage of the battery charger has an output error from a setting voltage, a reading voltage of the voltage measurement circuit of the battery pack also has a measurement error. Moreover, since resistive elements such as wirings, terminals or circuit elements are present between the output of the battery charger and the battery cell, the output voltage is not identical to the cell voltage, and the difference varies with charging current. In addition, in the charging system, there are some cases where the cell voltage changes abruptly with a change in ambient temperature or drift of the battery charger. When charging is normally performed on the battery cell, it is necessary to provide a sufficient margin between the setting value of the battery charger and an overvoltage setting value of the overvoltage protection circuit so that the overvoltage protection circuit of the battery pack does not malfunction.
In the past, in order to prevent malfunctioning of the overvoltage protection circuit, a gap is provided between the setting voltage and the overvoltage setting value so that an upper limit of the output voltage that varies with an output voltage error of the battery charger from the setting voltage does not overlap with a lower limit of the cell voltage measured by the voltage measurement circuit with a measurement error. For example, if the voltage measurement circuit has a reading error of ±0.05 V, the overvoltage protection circuit operating at an overvoltage setting voltage will be operated with the same error with respect to the cell voltage. Therefore, if the overvoltage setting voltage is 4.35 V, the minimum value of operation will be 4.30 V while a guaranteed value of the overvoltage protection circuit will be 4.40 V. In addition, when the battery charger has an output error of ±0.03 V, for example, and the setting voltage is 4.20 V, the upper limit of the output voltage will be 4.23 V. Therefore, a margin of 0.07 V can be provided between the lowest operating voltage (4.30 V) of the overvoltage protection circuit and the maximum output voltage (4.23 V), and thus the overvoltage protection circuit is prevented from malfunctioning during the charging process.
In recent years, lithium ion rechargeable batteries are demanded to provide a higher safety level. For this reason, related business groups are moving to further tighten the safety standards of the lithium ion rechargeable batteries. Specifically, in the past, the standard guaranteed value of the overvoltage protection circuit was 4.40 V; however, the standard is now under consideration for revising it to 4.25 V. Even when the corrected value of the overvoltage protection circuit is changed to 4.25 V, it is necessary to set the battery charger so that malfunctioning of the overvoltage protection circuit is prevented as long as the cell voltage is in a normal range.
In this case, the decreased guaranteed value can be dealt with by decreasing the overvoltage setting value and the setting voltage by the decreased amount (0.15 V). However, when the setting voltage of the battery charger is decreased, the full charge capacity is also decreased or the time to reach the full charge capacity is increased, which is not desirable. Even when the voltage measurement circuit used in the feedback control of the battery charger is corrected with the cell voltage received from the battery pack, it is impossible to completely absorb the output error of the battery charger. Moreover, it is necessary to provide a considerable gap between the overvoltage setting voltage and the setting voltage in order to deal with a change in the cell voltage resulting from drift of the battery charger, the resistive elements between the battery charger and the battery cell, and a change in ambient temperature. Therefore, it is difficult to set the setting voltage of the battery charger to the same level (4.20 V) as the case when the corrected value was 4.40 V.
Consequently, it would be desirable to provide a charging system that can maintain a high-setting voltage of a battery charger while maintaining a low-setting value of an overvoltage protection circuit with respect to a battery cell.